German Patent Publication DE 3,635,302 Al (Dietz et al.), published on Apr. 28, 1988, discloses a cooling system for extruded melt film in which two separate baths holding cooling liquid are arranged in series so that the film first passes through a first bath in which a cooling roller rotates and then through a second bath through which the film is guided by a plurality of guide rollers. Each bath has a separate cooling liquid circulation system. The liquid level in both baths is substantially the same. In another embodiment the two bath containers communicate with each other, but the water level is still the same in both containers. Nozzles may supply the cooling medium, normally water, into the first bath container in which the outwardly facing surface of the melt film is cooled. In the second bath both surfaces of the melt film are being cooled.
The structure of the cooling bath in German Patent Publication DE 3,635,302 Al is such that the extruded film initially, that is immediately after its contact with the cooling roller, is only inadequately cooled until the film immerses into the first bath. This feature makes it difficult to intensively cool the film near the extrusion area so that it will have a low stretchability that is required for further treatment of the film. For that reason the above mentioned second bath is arranged downstream of the first bath as viewed in the feed advance direction of the film. The sequential arrangement of two baths is considered to be a disadvantage because it takes up substantial floor space in the feed advance direction of the film.
In cooling systems of this type it is customary to distinguish between systems in which the bearings or mountings for the cooling roller are mounted above the water level in the respective bath and other systems in which the bearings or mountings for the cooling roller are immersed below the water level in the respective bath. The aim of the second type of mounting is to achieve a maximum cooling efficiency with a cooling roller that has a diameter as small as possible. The efficiency of the cooling effect increases the deeper the cooling roller is immersed in the water, because the melt film is contacted by the cooling water quickly after its exit from the die or nozzle. As a result, more melt film can be supplied to further treatment in a film producing system, thereby increasing the efficiency or throughput of the entire system in terms of thermoplastic film length produced per time unit.
The water level in the two baths according to German Patent Publication DE 3,635,302 Al, however, cannot be increased as much as would be desirable under the given geometrical conditions of the two baths arranged in series with each other. Further, so-called air doctor blades are necessary to clean the cooling roller surface to make it ready again for receiving melt film and for squeezing the melt film against the cooling roller in order to avoid air inclusions between the melt film and the cooling roller. Such air doctor blades comprise one or more air nozzles that direct a shaped airstream against the cooling roller and/or against the film surface. Normally, there is sufficient space for positioning air doctor blades for squeezing the melt film against the surface of the cooling roller. However, space for placing cleaning elements including air drying nozzles is limited to the area or sector of the cooling roller surface between the extrusion nozzle and the film take-up roller. Yet, it is necessary to clean and dry the cooling roller surface in the just described area upstream of the nozzle and downstream of the take-up roller to remove any remaining water and to dry the cooling roller surface for receiving melt film. These additional cleaning and drying components also require a certain space for their installation relative to the cooling roller. For an efficient cooling the area in question should be kept as small as possible so that as much cooling roller surface as possible can be immersed in the cooling water. These two requirements, namely providing enough mounting space for cleaning and drying elements and simultaneously an optimal roller immersion are in conflict with each other, because for an efficient cooling one wants to make the looping angle of the melt film around the cooling roller surface as large as possible, while for the cleaning purpose one would like to increase the roller surface area not covered by melt film to properly position the respective cleaning and drying components such as blow nozzles and squeegees including squeegee rollers for first removing most of the water from the cooling roller surface and then drying that surface with a high pressure airstream that blows any remaining water off the surface, preferably laterally, so that no water comes into the area near the extrusion nozzle.
A relative efficient water removal from the surface of the cooling roller is achieved when that surface encloses with the horizontal a relatively large inclination angle which is the case for surface portions away from the nozzle. The angle gets smaller the closer a particular roller surface area approaches the nozzle position. When the inclination angle is large, entrained water tends to flow back by gravity into the bath. However, when the cooling roller is immersed into the cooling bath to a point above the roller center axis, the sector that remains available for cleaning becomes rather small and the inclination also becomes small, which makes it more difficult to clean and dry the roller surface area between the die or nozzle and the film take-up roller. This problem is aggravated when the cooling roller rotates with substantial r.p.m.s because more water is entrained and its removal becomes more difficult. The entraining of water onto the surface of the cooling roller becomes even more pronounced the smaller the above mentioned inclination angle is. Thus, it would be advantageous to have a substantial inclination angle in the surface sector where the cleaning of the roller surface must take place. The larger that sector becomes, the more room there is for the installation of cleaning and drying components.
European Patent Publication EP 0,172,924 B1 (Kobayashi et al.), published on Mar. 5, 1986 discloses a sheet forming apparatus in which first and second cooling rollers are arranged above a cooling bath. The position of the second cooling roller determines the looping or wrap angle of the film around the first cooling roller. The film then travels through a cooling liquid bath. Guide rollers submersed in the cooling liquid guide the film through the bath downstream of the two cooling rollers. Such a structure does not provide the desired intensive cooling of the film immediately after its extrusion, especially the outer surface of the film, even though the inner surface of the film that contacts the cooling roller may be sufficiently cooled. Thus, a film cooling nozzle is positioned near the nozzle. The second cooling roller that determines with its position the looping angle, starts cooling the outer surface of the film after the film has travelled through the looping angle. Still, a water bath is also required downstream of the two cooling rollers for cooling the film into a condition with a low stretchability suitable for further treatment of the film.
U.S. Pat. No. 5,049,223 (Dais et al.) relates to the use of surface active agents for improved cooling of thermoplastic film webs. U.S. Pat. No. 5,318,648 (Heyes et al. ) discloses laminating polyester film onto a metal substrate.
The above prior art leaves room for improvement, especially with regard to a compact arrangement of different baths.